What toothpaste can cause, however, is irritation or allergic reactions in people with certain sensitivities, resulting in rashy bumps around the mouth or, perhaps, rosacea, a chronic condition of redness and skin sores that might be confused with traditional acne, said Dr. Richard Gallo, chair of the dermatology department at the University of California at San Diego.

"One of the instructions we give to patients with rosacea is to think about the toothpaste you're using," Gallo said.

People can be allergic to just about anything, but toothpaste's mint and cinnamon flavorings, which can include the allergens balsam of Peru, cinnamic aldehyde and peppermint and spearmint flavors, are major culprits in skin reactions, Gallo said.

Trouble is, 95 percent of 153 toothpastes evaluated in a study did not list the specific flavoring ingredients, so people wouldn't know to avoid them, researchers wrote in a study published in September in the Journal of Clinical and Aesthetic Dermatology. And it's not easy to find toothpaste without any flavoring.

Other potential allergens that were listed in many of the toothpastes reviewed included sorbitan sesquioleate derivatives, propylene glycol, cocamidopropyl betaine, sodium benzoate and benzoic acid, the study found.

People who are worried they are allergic to toothpaste ingredients should see a specialist who can test reactions to the specific ingredients, Gallo said.

But sometimes a skin flare-up has nothing to do with an allergy and is instead irritation.

Sodium lauryl sulfate, a detergent responsible for the foaming properties of toothpaste, is a common irritant that can aggravate the skin around the mouth just as might happen when you eat citrus or spicy foods, said Dr. Andrew Scheman, a dermatologist at the North Shore Center for Medical Aesthetics in Northbrook, Ill., who specializes in contact allergies and was the lead author on the toothpaste study.

Some dermatologists have implicated fluoride as an irritant causing perioral dermatitis, a rash of tiny red bumps around the mouth usually brought on by topical steroids. But few studies have proved the link, and the claims remain "unsubstantiated," Gallo said.

As for whitening toothpastes, Scheman said that if they cause irritant or allergic reactions, it's likely because of something else in the toothpaste, not the whitening agents.

Super sensitive?

If you worry your toothpaste may be responsible for your skin reactions, try switching to Tom's of Maine natural toothpastes, which have all ingredients listed and explained at tomsofmaine.com. Some are made without sodium lauryl sulfate or fluoride.